Traceback (most recent call last):
  File "/usr/local/lib/python3.6/unittest/case.py", line 59, in testPartExecutor
    yield
  File "/usr/local/lib/python3.6/unittest/case.py", line 605, in run
    testMethod()
  File "/var/local/git/grpc/src/python/grpcio_tests/tests_aio/unit/_test_base.py", line 31, in wrapper
    return loop.run_until_complete(f(*args, **kwargs))
  File "/usr/local/lib/python3.6/asyncio/base_events.py", line 484, in run_until_complete
    return future.result()
  File "/var/local/git/grpc/src/python/grpcio_tests/tests_aio/interop/local_interop_test.py", line 73, in test_empty_stream
    self._stub, None)
  File "/usr/local/lib/python3.6/asyncio/coroutines.py", line 129, in throw
    return self.gen.throw(type, value, traceback)
  File "/var/local/git/grpc/src/python/grpcio_tests/tests_aio/interop/methods.py", line 449, in test_interoperability
    await method(stub)
  File "/usr/local/lib/python3.6/asyncio/coroutines.py", line 129, in throw
    return self.gen.throw(type, value, traceback)
  File "/var/local/git/grpc/src/python/grpcio_tests/tests_aio/interop/methods.py", line 247, in _empty_stream
    assert await call.read() == aio.EOF
  File "/usr/local/lib/python3.6/asyncio/coroutines.py", line 129, in throw
    return self.gen.throw(type, value, traceback)
  File "/var/local/git/grpc/py36_asyncio/lib/python3.6/site-packages/grpc/aio/_call.py", line 353, in read
    response_message = await self._read()
  File "/usr/local/lib/python3.6/asyncio/coroutines.py", line 129, in throw
    return self.gen.throw(type, value, traceback)
  File "/var/local/git/grpc/py36_asyncio/lib/python3.6/site-packages/grpc/aio/_call.py", line 331, in _read
    await self._preparation
  File "/usr/local/lib/python3.6/asyncio/coroutines.py", line 126, in send
    return self.gen.send(value)
  File "/var/local/git/grpc/py36_asyncio/lib/python3.6/site-packages/grpc/aio/_call.py", line 624, in _prepare_rpc
    self._metadata, self._metadata_sent_observer)
  File "src/python/grpcio/grpc/_cython/_cygrpc/aio/call.pyx.pxi", line 499, in initiate_stream_stream
  File "src/python/grpcio/grpc/_cython/_cygrpc/aio/call.pyx.pxi", line 164, in grpc._cython.cygrpc._AioCall._set_initial_metadata
TypeError: 'NoneType' object is not iterable